[PATCH] help: correct behavior for is_executable on Windows

From: Heiko Voigt <hvoigt@hvoigt.net>
Date: Aug 11, 2012, 07:00 UTC
Message-ID: <20120811070030.GA83665@book.hvoigt.net>

The previous implementation said that the filesystem information on Windows is not reliable to determine whether a file is executable. To find gather this information it was peeking into the first two bytes of a file to see whether it looks executable. Apart from the fact that on Windows executables are usually defined as such by their extension it lead to slow opening of help file in some situations.

When you have virus scanner running calling open on an executable file is a potentially expensive operation. See the following measurements (in seconds) for example.

With virus scanner running (coldcache):

$ ./a.exe /libexec/git-core/ before open (git-add.exe): 0.000000 after open (git-add.exe): 0.412873 before open (git-annotate.exe): 0.000175 after open (git-annotate.exe): 0.397925 before open (git-apply.exe): 0.000243 after open (git-apply.exe): 0.399996 before open (git-archive.exe): 0.000147 after open (git-archive.exe): 0.397783 before open (git-bisect--helper.exe): 0.000160 after open (git-bisect--helper.exe): 0.397700 before open (git-blame.exe): 0.000160 after open (git-blame.exe): 0.399136 ...

With virus scanner running (hotcache):

$ ./a.exe /libexec/git-core/ before open (git-add.exe): 0.000000 after open (git-add.exe): 0.000325 before open (git-annotate.exe): 0.000229 after open (git-annotate.exe): 0.000177 before open (git-apply.exe): 0.000167 after open (git-apply.exe): 0.000150 before open (git-archive.exe): 0.000154 after open (git-archive.exe): 0.000156 before open (git-bisect--helper.exe): 0.000132 after open (git-bisect--helper.exe): 0.000180 before open (git-blame.exe): 0.000718 after open (git-blame.exe): 0.000724 ...

This test did just list the given directory and open() each file in it.

With this patch I get:

$ time git help git Launching default browser to display HTML ...

real 0m8.723s user 0m0.000s sys 0m0.000s

and without

$ time git help git Launching default browser to display HTML ...

real 1m37.734s user 0m0.000s sys 0m0.031s

both tests with cold cache and giving the machine some time to settle down after restart.

Signed-off-by: Heiko Voigt <heiko.voigt@mahr.de>
---
 help.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/help.c b/help.c
index 662349d..b41fa21 100644
--- a/help.c
+++ b/help.c
@@ -103,10 +103,19 @@ static int is_executable(const char *name)
 		return 0;
 
 #if defined(WIN32) || defined(__CYGWIN__)
+	/* On Windows we cannot use the executable bit. The executable
+	 * state is determined by extension only. We do this first
+	 * because with virus scanners opening an executeable for
+	 * reading is potentially expensive.
+	 */
+	if (has_extension(name, ".exe"))
+		return S_IXUSR;
+
 #if defined(__CYGWIN__)
 if ((st.st_mode & S_IXUSR) == 0)
 #endif
-{	/* cannot trust the executable bit, peek into the file instead */
+{	/* now that we know it does not have an executable extension,
+	   peek into the file instead */
 	char buf[3] = { 0 };
 	int n;
 	int fd = open(name, O_RDONLY);
@@ -114,8 +123,8 @@ if ((st.st_mode & S_IXUSR) == 0)
 	if (fd >= 0) {
 		n = read(fd, buf, 2);
 		if (n == 2)
-			/* DOS executables start with "MZ" */
-			if (!strcmp(buf, "#!") || !strcmp(buf, "MZ"))
+			/* look for a she-bang */
+			if (!strcmp(buf, "#!"))
 				st.st_mode |= S_IXUSR;
 		close(fd);
 	}

-- 
1.7.12.rc2.10.gaf2525e